The environmental group's biennial Living Planet Report found the number of mammals, birds, reptiles, amphibians, and fish in the wild dropped more than 50% between 1970 and 2010.

Image: WWF

Jon Hoekstra, Chief Scientist at World Wildlife Fund said the population decline is far greater than researchers initially anticipated.

"This is the tenth of our Living Planets Report, so we have been tracking it for years, but the scientists who crunched the numbers updated their methods and found far more than expected," he said.

"The scale is jaw dropping. The fact that there is half as much wildlife in the world as there was when I was born is really staggering."

The report attributes most of the decline to human activity, including overfishing, habitat loss, climate change and other man-made factors. The study found humans are consuming resources at an unsustainable rate, using 1.5 Earths-worth of resources each year.

Scientists researched 10,380 separate populations of 3,038 species, and developed an indicator called the Living Planet Index to measure trends over time. The region that experienced the biggest losses was South America, where terrestrial wildlife has been reduced by 83% since 1970.

Freshwater species were hit the hardest worldwide, with animals including fish, ducks, geese, frogs and salamanders experiencing a decline of 76%.

Image: WWF

Stuart Pimm, a biology professor at Duke University and prominent expert in animal extinction, criticized the Living Planet Index for being too confusing to accurately interpret.

"What it shows is that the variety of life on earth is in trouble — and that broad conclusion is absolutely correct," Pimm told Mashable. "But the problem with this particular index is that it lumps all sorts of things together, and doesn't do it in a way that is terribly helpful."

Pimm says the study gives the public the false impression that 50% of animals have gone extinct in the last 40 years. He agrees that biodiversity is at risk, but says the index does not accurately quantify the losses over the years.

"Are there populations that are becoming scarcer? Yes. Does that mean they’re going extinct? No, they’re declining, they haven’t gone extinct, they’re just worse off than they were 40 years ago," he said.

Pimm said he thinks it would be more beneficial to focus on the top environmental problems — which he identified as species extinction, the depletion of tropical rain forests, poaching and overfishing.

"The [Living Planet Index] is an attempt to come up with a single number that the world can understand as a measure of what is happening to biological diversity," he said. "I understand the need to do that, but I think the general public are smart enough that they can understand a few different issues at once."

Given the current pace of decline, the WWF report warned, the "possibility of reaching critical tipping points" can no longer be excluded. But Pimm said the notion of such a tipping point is not supported by research.

"That is a complete fabrication," he said. "It’s an attempt to grab attention to a particular view of the world. There is simply no evidence that there is some point below which biodiversity cannot go."

Tipping points are, however, well-accepted possibilities when it comes to global warming — where a certain amount of global warming could cause a self-sustaining loss of the Greenland ice sheet, for example.

In an earlier statement, Hoekstra acknowledged that the study is dense. But he said the conclusions are clear: It is time to take action.

"There is a lot of data in this report and it can seem very overwhelming and complex," he said. "What’s not complicated are the clear trends we’re seeing — 39 percent of terrestrial wildlife gone, 39 percent of marine wildlife gone, 76 percent of freshwater wildlife gone –- all in the past 40 years."

Hoekstra said the WWF is hopeful that species decline can be reversed, but that it will take a widespread effort to make a change.

"This should come across as a real wake up call to us all, that the environmental problems we have all been aware of are far more serious than we thought," he said. "The time is now to take action if we are going to save wildlife, and save ourselves in the process."

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